32 bit iMac, will 64 bit run properly?
Blender dropped support for the 32bit versions of OSX in 2.72.
To your question about running a 64bit version on your 32bit system, I will direct you here.
If you find that you can not use any 64bit software, because of your CPU or some other reason; then you can still use 2.71 with no problem.
Download link for 2.71 32bit OSX on blender's server.